It is known to fix one or a plurality of implants into one or a plurality of osseous elements in order to connect to the skeleton a device implanted in the human or animal body, said implants being then used to fasten certain elements of said device. For maintaining or correcting the rachis in particular, use of an osteosynthesis device comprising one or a plurality of maintenance bars or plates positioned along the vertebral column and fixed to certain vertebrae by implants is well-known. These implants are fixed on the one hand to the bars and on the other to the vertebrae by an osseous anchoring means comprised of a hook having its support on a vertebra or of a threaded part screwed inside the vertebra, for example, at the pedicle. In the case of an osteosynthesis of the extreme vertebrae one or a plurality of implants can of course be securely fastened to adjacent bone, the sacrum for example.
The FR 0104717 patent discloses for this purpose an implant comprising an osseous anchoring part and a fixation head traversed by a channel where the bar is clamped. This document describes an implant, whose head comprises a lateral opening enabling the introduction of a bar with flat areas wherein a clamping screw has already been pre-installed in the superior part of said head. In order to assure satisfactory contact of the clamping screw on the flat surface whatever the angular position of said flat surface about the axis of the pin, the clamping screw is provided with a tiltable support surface mounted on a ball and socket joint.
By supporting itself on the inside shape of the channel under the effect of the clamping, the bar's position is thus definitively determined by the general position of the implant. At the time of attachment of the bar to an implant that is already anchored in the rachis, if the bar is not in a position corresponding to that of the channel, the act of forcing its introduction can induce a certain stress in the bar.
On the one hand, said stress can then make the attachment of the bar to the implant difficult or render its clamping not very accurate. This problem is true more particularly when the bar has already been engaged with a first implant and is being attached to a second implant. In order to reduce this stress, it is conceivable to anchor the implant according to the position of the bar, but this is not always possible to do nor easy to predict. It would also be conceivable to deform the bar, which could be a problem in the situation and plays against the requirement of a rigid bar to assure an effective hold. This stress can thus make manipulation delicate to execute, in particular in weale intrusion surgical procedures, for example video-assisted or laparoscopic procedures.
On the other hand, even if clamping enables stressing the bar to adopt a position or a shape corresponding to the implant, said stress will persist permanently over a very long time after the procedure. The fact that the bar is under permanent stress poses the risk of a mechanical effect directly on the rachis, for example causing pains or changing or disturbing the correction or the support sought by using the osteosynthesis device.